Conventionally, external components located on invasive medical catheters, such as an electrode or thermocouple, are manually placed on the catheter body--e.g., in the form of wound coil or a conductive band. The components are typically held in position with an adhesive, a process which is relatively time consuming and expensive.
One notable problem with this construction is that if an external component is not properly fit onto the catheter body, small openings and crevices at the edges of the component may be formed, allowing for the ingress and retention of bodily fluids or tissue during use. Additionally, this traditional catheter-electrode construction can result in undesirably high electrode edge effects caused by the sharp transition between the conductive electrode band and the immediately adjacent non-conductive catheter body.
Further, because the electrode components come into direct contact with a patient's blood stream and body tissues during use, non-biocompatible (i.e., toxic) materials otherwise having advantageous characteristics for use in an externally mounted electrode, including silver or lead, cannot be used.
Therefore, a need has existed for an improved external electrode constructions for invasive medical catheters.
Such improvements are disclosed and described in U.S. patent application Ser. No. 08/879,343, filed Jun. 20, 1997, now U.S. Pat. No. 5,991,650, which is fully incorporated herein by reference for all it discloses and describes. As disclosed therein, a metal-based conductive ink is used to form exterior electrodes on a non-conductive polymer catheter tubing by processes such as pad printing, vapor deposition, ion beam assisted deposition, electroplating or other printed circuit manufacturing processes. Preferred ink materials include a silver/silver chloride filled polyurethane composite ink that is flexible and highly electrically conductive after the polyurethane is cured. The printed ink electrodes are then covered with an electrically conductive outer coating, preferably formed from a material comprising regenerated cellulose.
The regenerated cellulose coating secures the underlying electrode structures onto the catheter, while still enabling electrical contact between the electrodes and surrounding body tissue structures. One advantage of using regenerated cellulose for the protective coating is that regenerated cellulose is ion-permeable, thereby allowing ionic transfer of electrical energy from the electrodes into the patient's bloodstream and/or body tissue, while preventing macromolecules, such as blood proteins, from contacting the printed electrode material during use.
Additionally, because the regenerated cellulose surface coating produces a smooth outer surface to the distal end assembly, lead wires and temperature sensing devices can be bonded to the exterior surface of electrodes and then coated to produce a smooth outer surface, thus providing a simple, inexpensive manufacturing method for the attachment of such components to the electrodes.
In particular, the regenerated cellulose coating acts as a mechanical barrier between the catheter components, such as electrodes, preventing ingress of blood cells, infectious agents, such as viruses and bacteria, and large biological molecules such as proteins, while providing electrical contact to the human body. As a result the electrodes can be made using more efficient processes (such as pad printing) that have been previously rejected due to lack of robustness when directly exposed to bodily tissues on a catheter surface.
The regenerated cellulose coating also acts as a biocompatible barrier between the catheter components and the human body, whereby the components can now be made from materials that are somewhat toxic (such as silver or copper), because the diffusional distance to tissues is increased substantially, and because a lower percentage of the metallic surface is exposed (both directly and indirectly) to the tissue.
In addition, coating electrodes with regenerated cellulose decreases the effect of convective cooling on the electrode during RF energy delivery. That is, since regenerated cellulose is a poor thermal conductor when compared to metal, the effect of convective cooling by blood flowing past the regenerated cellulose coated electrodes is diminished. This provides better control for the lesion-generating process because the hottest tissue temperature is closer to the ablation electrode.
Furthermore, the regenerated cellulose coating decreases the edge effects attributed to delivering RF energy to the electrode having sharp transition between the conductive electrode and insulating catheter tubing. The current density along the electrode and power density within tissue are more uniform, which reduces the incidence and severity of char and/or coagulum formation. The more uniform current density along the axis of the catheter also results in a more uniform temperature distribution at the electrode, which decreases the requirement for precise placements of the temperature sensors at the ablation electrodes.
Notably, intimate contact between the regenerated cellulose coating and the conductive electrodes on the catheter body is required to ensure reliable pacing, electrogram sensing, or ablation through the microporous structure of the regenerated cellulose coating. While the regenerated cellulose coating closely conforms to the catheter body, e.g., like a skin, it does not actually adhere to the polymers commonly used to make catheters, such as, e.g. polyether block amides (PEBAs). Nor does it adhere to metal-based printed ink materials. Instead, a mechanical fit of the regenerated cellulose "jacket" on the distal end of the catheter is relied upon.
Thus, if the distal end of the catheter is aggressively torqued or twisted, the regenerated cellulose jacket can at times "barber pole" or become axially wrinkled or blistered, resulting in a loss of direct contact of the coating and the underlying electrode structure. This can result in poor, intermittent, or even loss of electrical contact with the electrode. This compromised electrode contact can result in noisy recordings, inconsistent pacing thresholds, and unpredictable (and therefore uncontrollable) ablation conditions, depending upon the particular application.
Further, if the catheter is introduced through a close-fitting introducer (e.g., such as a pre-shaped guide sheath), the regenerated cellulose coating can become stretched axially relative to the underlying catheter body structure. Because there is no conductive fluid such as saline between the electrode and regenerated cellulose coating, the electrical path can become intermittent or open where the two materials become separated.
Thus, for reasons of durability, consistent signal quality, pacing capabilities and ablation, it would be beneficial to improve upon the disclosure of U.S. patent application Ser. No. 08/879,343, and provide a coherent composite assembly for protecting the distal portion of the finished catheter device.